1. Field of the Invention
The present invention relates to an imaging device, either for ionizing radiation such as X or gamma rays or charged particles, or for light radiation, whose wavelength can be in the visible, ultraviolet or infrared. More specifically, the invention applies to non-focusable ionizing radiation.
2. Description of the Prior Art
The imaging or taking of images of an ionizing radiation makes it possible by radiographic projection or tomographic reconstitution to study living or non-living matter and consequently carry out a non-destructive analysis of the internal structure of objects having variable sizes and masses and in particular in medical and nuclear fields, but also in industry. This device can be of a large size, namely having a surface area of a few dozen to a few hundred cm.sup.2, mainly for mamography, or of a small size, namely a few hundred mm.sup.2, e.g. for intra-buccal dental imaging. In the latter case, the device can have a total thickness of 3 mm. This compactness is very important so as not to be prejudicial either to the patient or to the dentist.
An imaging device has in known manner a radiation detecting part constituted by one or more detectors transforming the incident radiation into electrical charges and having electrodes for collecting these charges and a part for treating the electric signals resulting from the collection of the charges and constituted by integrated circuits for amplifying said electric signals and an external device for the processing and storage of the signals supplied by said integrated circuits. In addition, each detector is structured in the form of elementary display points or pixels arranged in accordance with the rows and columns of a matrix.
In a first type of detector use is made of a semiconductor material equipped with point electrodes distributed in matrix form. In this type of detector, the electrical connection of the reading circuits to each detector is provided in known manner by pixelwise microsphere hybridization.
Such a system is described by M. Campbell et al (A 10 MHz micro power CMOS front-end for direct readout of pixel detectors, IEEE Nuclear Science Symposium USA, 1989, pp 1-8) and in EP-A-415 541 and EP-A-462 345.
In another detector type use is made of charge coupled devices (CCD) which are directly irradiated. The irradiation of these CCD's leads to a premature ageing, which brings about a sensitivity loss and defective pixels.
In dental radiography, the X-rays used have a high energy of 30 to 60 keV. In addition, the imaging device as a limited thickness of generally 300 to 500 .mu.m, so that it only has an X-ray stopping power of a few per cent (&lt;10%). Therefore 90% of the X-photons (and therefore the dose) are needlessly imposed on the patient, which is a major disadvantage.
For mamography use is made of X-rays with an energy of 15 to 25 keV.
For producing large imaging devices, it is known to use several matrixes or arrays of detectors joined side by side as described in EP-A-245 147 or arranged in the manner of the tiles of a roof as described in EP-A- 421 869.
In EP-A-245 147, the detector is a thick scintillator transforming the ionizing rays into visible light and associated with a photodiode structured as pixels. These imaging devices generally lead to images having shadow areas and information losses.